1. To demonstrate or to prove that something is just, right, or valid: "There are some people who will commit illegal or indecent acts for a something that they believe in because they argue that the end justifies the means."
2. To declare free of blame; to absolve: "Eddie tried to justify his poor grade on the exam by claiming that he wasn't feeling well."
3. To demonstrate sufficient legal reason for an action taken.
4. In printing or typing text on the computer: "To adjust or to justify the spacing within lines in a document; for example, so that the lines end evenly as straight margins."